This invention relates to a method for stabilizing the mean-frequency and modulation slope of a phase-keyed frequency-controlled oscillator.
In information technology the employment of multistep phase-modulation systems (PSK systems) is very extensive. Such PSK signals are usually asynchronously demodulated with the help of phase-difference discriminators. These demodulators are simple in construction and are relatively insensitive to small fluctuations in the mean frequency and in the phase deviation of the PSK signal. However, since such systems exhibit a much lower signal-to-noise ratio than synchronous demodulation systems, it is preferable to use synchronous, orthogonal demodulation in connection with heavily distorted PSK systems, or in systems with high demands on the quality of transmission. This latter method of demodulation requires that the phase of the carrier signal be accurately controlled since the transmitted data signal is deduced from the instantaneous difference between the carrier phase and the phase of the PSK signal. While PSK systems transmit information as a function of the instantaneous phase shift of the transmitted signal, the actual phase modulation is effected by control of the frequency. Accordingly, it is necessary to accurately regulate the mean frequency of the transmitted signal as well as the modulation slope to minimize any variations in the actual instantaneous phase of the transmitted signal and the desired phase thereof, in order to prevent the build up of a phase error over a long persistent keying in the same sense.
The method according to the invention for stabilizing the mean frequency and the modulation slope of an oscillator frequency modulated for phase keying is suitable for the generation of a "hard" keyed PSK signal of any number of steps.
According to the present invention there is provided a method of stabilizing the mean frequency and the modulation slope of an oscillator frequency-controlled for phase-keying, wherein the output signal of a monitor discriminator arranged to respond to the phase deviation of the frequency controlled oscillation with respect to a reference oscillation is utilized both to correct the oscillator frequency and, after polarity inversion by the signal effecting the phase keying to control the modulation slope.